The present invention relates generally to pressureless sintering of silicon nitride (Si.sub.3 N.sub.4) compositions. It relates particularly to pressureless sintering of such compositions at temperatures of 1650.degree. Centigrade (.degree.C.) or below. It relates more particularly to pressureless sintering at such temperatures of Si.sub.3 N.sub.4 compositions that include bismuth oxide (Bi.sub.2 O.sub.3) and, optionally, zirconium oxide (ZrO.sub.2) as part of a sintering aid combination.
Silicon nitride ceramics are recognized for their excellent mechanical and physical properties, including good wear resistance, low coefficient of thermal expansion, good thermal shock resistance, high creep resistance and high electrical resistivity. In addition, Si.sub.3 N.sub.4 ceramics resist chemical attack, particularly oxidation. These properties or attributes make Si.sub.3 N.sub.4 ceramics particularly useful in a variety of wear and high temperature applications, such as cutting tools and parts in pumps and engines.
Silicon nitride has two characteristics, covalent bonding and low diffusivity, that pose problems for those seeking to convert Si.sub.3 N.sub.4 powder to a high density part with little or no porosity. The problems are particularly severe in the absence of densification aids, applied pressure at elevated temperatures as in hot pressing or both.
Hot pressing Si.sub.3 N.sub.4 powder or preformed parts (also known as greenware) typically yields simple rectangular or disc-shaped articles. These articles must then be converted to a desired shape by various procedures that may be costly, slow or quite difficult because dense Si.sub.3 N.sub.4 bodies have a high degree of hardness. These procedures, which include grinding, sawing and drilling, generally do not produce intricate shapes. Largely because of limitations such as cost and limited availability of shapes, many efforts focus upon pressureless sintering as a preferred densification route.
U.S. Pat. No. 4,264,547 (de Pous) discloses Si.sub.3 N.sub.4 -based sintering compositions containing Si.sub.3 N.sub.4 and no more than 6% by weight (wt-%) of very finely ground MgO and Al.sub.2 O.sub.3 as densification aids. The Si.sub.3 N.sub.4 has a particle size of less than 1 micrometer (.mu.m). The MgO has a particle size of 0.05 to 0.1 .mu.m. The Al.sub.2 O.sub.3 has a particle size that is less than that of the Si.sub.3 N.sub.4. The MgO and Al.sub.2 O.sub.3 are present in a weight ratio that lies between 10:1 and 1:3. Pressureless sintering conditions include heating in a nitrogen atmosphere for 2 to 20 minutes at 1650.degree. to 1830.degree. C.
J. Barta et al., in "Pressureless Sintering of Silicon Nitride", Science of Ceramics, Vol 11, pages 219-224 (1981), describe sintering combinations of Si.sub.3 N.sub.4, MgO, Al.sub.2 O.sub.3 and, optionally, SiO.sub.2 under 1.3 atmospheres (0.13 MPa) at 1550.degree. C. and 1650.degree. C. for one hour. They note that attrition milling is needed to obtain high densities at relatively low temperature. The MgO, Al.sub.2 O.sub.3 and, optionally, SiO.sub.2 are present in a total amount of about 10 wt-%, based upon total combination weight.
R. W. Dupon et al., in "Low-Temperature Route to Cordierite Ceramics Using a Reactive Liquid Phase Sintering Aid, Dense Body Preparation and Green Tape Fabrication", Materials Research Society Symposium Proceedings, Vol. 154, pages 351-356 (1989), teach that bismuth oxide (Bi.sub.2 O.sub.3) is a useful flux for preparing a dense body of cordierite (Mg.sub.2 Al.sub.4 Si.sub.5 O.sub.18). They also teach that the microstructure of a sintered body formed from 92 wt-% cordierite and 8 wt-% Bi.sub.2 O.sub.3 has residual Bi.sub.2 O.sub.3 flux in discontinuous domains at grain boundaries and triple points. Reaction of Al.sub.2 O.sub.3, MgO and SiO.sub.2 to form cordierite and subsequent densification of the cordierite occurs in the presence of 2 to 10 atomic percent bismuth over a 12 hour period at 1000.degree. C. At less than 2 atomic percent bismuth neither the reaction nor densification is complete. They achieve homogeneous distribution of bismuth ions via a precipitation technique.
JP (Kokai) Number 2-263763 describes sintering a molded material consisting of either 0.5-20 wt-% Bi.sub.2 O.sub.3 powder or a combination of 0.5-10 wt-% Bi.sub.2 O.sub.3 powder and 0.5-10 wt-% Al.sub.2 O.sub.3 powder and the balance Si.sub.3 N.sub.4 powder at a temperature between 1600.degree. C. and 2100.degree. C. under an added pressure of 1 to 2,000 atmospheres (0.101 to 202 MPa). The Bi.sub.2 O.sub.3 acts as a sintering aid and remains in the sintered material by forming a highly viscous glass phase located at grain boundaries. Combinations of Bi.sub.2 O.sub.3 and Al.sub.2 O.sub.3 allow sintering at lower temperature and pressure than Bi.sub.2 O.sub.3 alone. The reference teaches that Bi.sub.2 O.sub.3 enhances the toughness of the glass phase and allows the sintered body to retain its strength at high temperatures. The reference also teaches that a post-sintering heat treatment to crystallize the glass phase is unnecessary.